Early detection and treatment of cerebral ischemia to prevent further neurological damage in patients with severe brain injuries belongs to the most important issues in Neurocritical Care. Further, during neurological and neurologically related surgical procedures it is often desirable to continuously monitor the oxygenation of blood which is supplied to the brain. Near infrared spectroscopy (NIRS) is used for a wide variety of applications including invasive and non-invasive monitoring of cerebral blood flow (CBF) and cerebral oxygenation pattern, i.e. static and dynamic characteristics of cerebral blood and blood flow. The NIRS measurement of blood parameters is based upon the finding that light in the near infrared region penetrates biological tissue and is absorbed and scattered differently by hemoglobin chromophores in the deoxygenated and oxygenated state. Further, the concentration and flow of tracers such as the dye indocyaningreen (ICG) injected in the blood can be measured by NIRS to obtain information on parameters of cerebral hemodynamics, especially cerebral blood flow (CBF), mean transit time of ICG and oxygen metabolism. In pulse oximetry the temporal behaviour of NIRS signals is evaluated to obtain information about the fraction of oxygenated hemoglobin in the arterial blood. Other parameters are the concentration of deoxygenated and oxygenated hemoglobin, the mean transit time, the cerebral blood volume (CBV), cerebral blood flow (CBF) and the tissue oxygen index (TOI). The measurement and evaluation of the aforementioned parameters with NIRS are described in Jobis, F. F., “Noninvasive infrared monitoring of cerebral and myocardial oxygen sufficiency and circulatory parameters”, Science 198; 1264–1267 and I. Roberts, P. Fallon, et al., “Estimation of cerebral blood flow with near infrared spectroscopy and indocyaningreen”, Lancet 342; 1425.
Non-invasive techniques, e.g. as described in U.S. Pat. No. 4,223,680 or U.S. Pat. No. 5,218,962, use NIRS optodes placed on the head. To obtain information on the chromophores oxyhemoglobin and deoxyhemoglobin in cerebral vessels the detected NIRS signal gained by non-invasive techniques has to be corrected for effects due to light reflection and scattering by and in extracerebral tissue, i.e. skin and bone. The apparatus described in U.S. Pat. No. 4,223,680 therefore comprises a reference detector which detects light reflected or scattered back to the location of the light emitting optode. The reference signal is then used to correct the measured intensity for extracerebral tissue effects. The apparatus of U.S. Pat. No. 5,218,962 comprises two light emitting elements directing light through different regions of tissue and a photodetector detecting light travelling through both regions. The difference of the measured intensities represents how much the oxygen saturation of the first region differs from the second region, i.e. only relative blood parameters can be obtained. Due to the need for correction for extracerebral tissue effects non-invasive techniques are able to provide indirect information on blood parameters only.
With invasive techniques direct access to the brain and elimination of extracerebral contamination is gained through a burr hole in the skull, and a sensor which optically measures oxygenation without artifacts caused by skin and bone can then be inserted through such a burr hole. A sensor capable of monitoring several parameters instantaneously is disclosed in U.S. Pat. No. 5,916,171. Several signal guides for electrical signals and a single light guide are arranged in a housing which is inserted in a burr hole having approximately the same diameter as the housing. The light guide and the electrodes terminate vertically at the brain tissue. UV and red light is coupled into the single light guide to measure relative changes of the blood flow velocities by analyzing the signal reflected back into the same light guide using Laser Doppler flowmetry. With this arrangement only relative parameters of flowing blood can be analyzed as the signal coming from static tissue components are not detectable in Laser Doppler flowmetry. Furthermore by Laser Doppler flowmetry only values of very small areas (about 1 mm2) are obtained. Futher, the probe is merely inserted into the burr hole and stabilized by the skull bone which can lead to brain injuries or artifacts in the measurements when the patient moves. It is therefore not suited for a long-term measurement. Monitoring regions of tissue other than those of the burr hole is not possible. As the probe comprises a complex arrangement of a plurality of sensors its manufacturing costs are high and it is therefore not suited as a throw away article. Products that contact the brain, however, should be throw away articles as sterilizing is often not sufficient to exclude a potential infection risk.
A sensor for measuring cerebral oxygen availability epidurally, i.e. between dura and skull bone, by optical reflectance is disclosed in U.S. Pat. No. 5,024,226. A pair of light emitting diodes (LED) and a photodetector are encapsulated by a coating and connected electrically to a power supply respectively a signal analyzer by a flexible wiring. The sensor tip including the diodes and the photodetector is inserted through a burr hole in the skull and maneuvered between dura and skull bone to a region chosen for the measurement.
It is therefore an object of the present invention to provide a probe and an apparatus for measuring absolute values of regional cerebral flow and cerebral oxygenation through a burr hole in the skull by optical reflectance which can be manufactured at relatively low cost and is therefore suited as a throw away article.